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SSDs do die, as Linus Torvalds just discovered

September 12, 2013 02:29 PM ET

Also, there are several flavors of NAND flash: single-level cell NAND writes just one bit per transistor, giving it innately greater performance and endurance; multi-level cell flash writes two bits per cell, which wears memory out more quickly; and most recently, 3-bit or triple-level cell flash has added yet another bit to the equation, further degrading native endurance.

For example, Samsung's 840 EVO SSD uses TLC memory, yet because of the sophistication of the controller chip and its software, it will outlast any other component of the laptop or desktop it's in, according to Chris Geiser, senior product manager of Samsung's Memory and Storage Division.

"If I'm writing 10GB a day to a 120GB SSD, it will last over 10 years," Geiser said.

Unlike hard disk drives, all SSDs slow down after initial use because once a sufficient amount of data has been written to them, the processor in the drive begins to move data around -- a function known as the read-modify-erase-write (erase-write) cycle. So each time new data is written to the SSD, data must first be marked for deletion before new data can be written. Over time, the cells or transistors in NAND flash wear out due to the erase-write cycle.

SSD makers have increased the sophistication of error correction and 'wear leveling' software, which works to more evenly spread data writes across a drive so as to not "wear out" any block of cells more quickly than another. But, eventually they all wear out.

The SSD controller and the firmware is where error correction code (ECC) and wear-leveling take place. In general, Janukowicz said, both the controller and firmware are what differentiate an SSD and its performance/reliability compared to a USB thumb drive, which also uses NAND flash.

The sophistication of the SSD controller, such as the digital signal processing algorithms used and the level of ECC, helps mitigate some of the intrinsic challenges of NAND, Janukowicz said.

As NAND process shrinks in size -- that is, as the transistors become smaller and smaller to accommodate greater density and capacity -- firmware must compensate for the increase in errors. (The smaller cells or transistors get, the more likely data errors will occur.) NAND flash process technology has shrunk from 35 nanometers (nm) a few years ago, to under 19nm today.

"From the data I've seen, client SSD annual failure rates under warranty tend to be around 1.5%, while HDDs are near 5%," Chien said.

So the bottom line is that SSDs will fail -- even if you're Linus Torvalds -- but they are still more reliable and much faster than hard disk drives.

covers storage, disaster recovery and business continuity, financial services infrastructure and health care IT for Computerworld. Follow Lucas on Twitter at Twitter @lucasmearian or subscribe to Lucas's RSS feed Mearian RSS. His e-mail address is lmearian@computerworld.com.

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